Multiple Modes of Electromigration Failure in SAC Solder Alloys

Abstract

The anisotropic nature of tin, the principle component of Pb-free solder, can cause significant variations the physical characteristics of the solder bumps. Grain size and orientation in a solder bump can drastically change the properties of one bump, giving rise to multiple potential failure modes between bumps. Electromigration may take place via grain boundary diffusion in one bump and interstitial diffusion in another. The greater the number of bumps in a test structure, the greater the probability of early failure resulting from large grains oriented in a direction that enables fast diffusion. The lognormal probability of electromigration failures can be expected to follow a linear trend. Deviation from this linear trend for the earliest failures suggests multiple modes of electromigration failure in the solder bump daisy chain.